This invention relates generally to the field of miniature electronic components or devices, particularly surface-mounted devices. More specifically, the present invention relates to that class of such devices in which a component is carried on a substrate and enclosed within a housing that is attached to the substrate. An example of such a device would be a miniature, surface-mounted, trimming potentiometer.
In many miniature electronic devices, such as surface-mounted trimming potentiometers, an electronic component is mounted or formed (e.g., by thick film printing) on the surface of a ceramic substrate, along with its associated conductive paths and termination pads. Terminal leads, for mounting the device on a printed circuit board, are attached (as by soldering) to the termination pads, and the surface of the substrate carrying the component is enclosed within a housing.
It is frequently necessary to attach the housing to the substrate in such a way as to form an hermetic seal around the components on the substrate. Typically, this has been accomplished by using an epoxy cement to bond the housing to the substrate. A significant disadvantage to this attachment method is that the housing and the substrate must be mechanically held together while the cement cures. Moreover, different temperature coefficients of expansion among the cement, the plastic of the housing, and the ceramic of the substrate can result in a loss of seal integrity when the device is exposed to high temperatures.
The prior art has addressed these problems by employing purely mechanical means to attach the housing to the substrate, and to provide the necessary hermetic sealing. One such arrangement, using a combination of clips to hold the housing and substrate together, and an internal O-ring for hermetic sealing, is disclosed in U.S. Pat. No. 4,626,823 to Smith. In the Smith device, terminal leads having upwardly-extending extensions are soldered to the substrate. The extensions are directed generally perpendicularly to the substrate, and they terminate in inwardly-bent tabs which engage slots in the top of the housing. The tabs are crimped downwardly into the slots to lock the housing and the substrate together. This locking of the housing to the substrate compresses an O-ring inside the housing against the substrate, thereby providing an hermetic seal around the components on the substrate.
While clip-type mechanisms, such as that of the Smith patent, have been successfully used in certain applications, the increasing use of subminiature devices, i.e., those with horizontal dimensions in the range of 3-4 mm, makes further simplification of the mechanical structure of such devices highly advantageous. Moreover, there is a constant search in the electronic component industry to improve the product in such a way as to lower its manufacturing cost.